The present invention generally relates to retroreflective articles, and, more specifically, to cube corner retroreflectors having selectively reduced visibility range.
Retroreflective articles are well-known for applications such as highway signs, safety reflectors, and road markers. Generally, cube corner versions of these articles have a frontal lens of clear, colored or uncolored resin, such as methyl methacrylate, with a smooth front surface and a plurality of retroreflective cube corner elements on the rear surface. The cube corner elements each have three reflecting faces.
Light from a remote source passes through the smooth front surface, reflects off each of the three faces of a cube corner element, and passes again through the front surface. In a perfect retroreflector, this light is returned in a direction exactly opposite to the incoming direction of light. Primarily because of imperfections, either accidental or by design, the reflected light is not returned only in a direction exactly opposite to the incoming direction, but rather is returned typically into a spreading pattern, centered on the exact return direction. This imperfect return reflection is still termed “retroreflection”. The spread retroreflected light enables the retroreflector to be visible from directions slightly away from the light source.
For example, if headlights from an automobile are the source of light, then the perfect retroreflective pavement marker would reflect light back only toward the headlights. It is desirable that the reflected light from a retroreflective pavement marker be seen by the driver of the automobile, whose eyes are generally higher than and somewhat left or right of the headlights.
Changes to the size or shape of the faces of the cube corner prism elements, or to the angles between the faces (dihedral angles), or to the flatness of the faces or the flatness of the front surface, can all change the pattern of retroreflection and thereby determine the regions around the light source in which the retroreflector visible. “Study of Light Deviation Errors in Triple Mirrors and Tetrahedral Prisms”, J. Optical Soc. Amer., vol. 48, no. 7, pp. 496-499 (July 1958) by P. R. Yoder, Jr., describes spot patterns resulting from the angles between faces being not exactly right angles. U.S. Pat. No. 3,833,285, to Heenan, which is incorporated in its entirety herein by reference, teaches that having one dihedral angle of a macro-sized cube corner element greater than the other two results in extended observation angularity in macrocubes, and specifically that the retroreflected light diverges in an elongated pattern. This elongated pattern has a generally substantially vertical axis. U.S. Pat. No. 4,775,219, to Appeldorn et al., teaches redistribution of the reflected light so that more light is directed to the driver of approaching vehicles or extending the pattern of light by modifying the dihedral angles of micro-sized cube corners.
The angle formed between the source, the retroreflector, and the observer is called the observation angle. Conventional pavement markers and other retroreflective articles are generally designed to be highly visible at long distances, corresponding to small observation angles. Because of imperfections, generally accidental, in conventional retroreflective articles, they are also highly visible at middle and close distances, corresponding to medium and large observation angles. For each type of retroreflector in each application, the relative value of long, medium, and close visibility may differ. Some researchers have suggested that long distance visibility of pavement markers might not be useful, or even have negative value.